1. Field of the Invention
The present invention relates to a digital receiving antenna device and, more particularly, to a digital receiving antenna device for a digital television.
2. Description of Related Art
Digital televisions require a digital receiving antenna device to receive digital program signals so the televisions can display the video and play the audio. For portable digital televisions, the digital receiving antenna device is particularly important to provide good quality of the program video.
With reference to FIG. 5, a small conventional digital receiving antenna device connects to the digital television (not shown) through a coaxial cable (60) and has a casing (50), a flat antenna (51) and first and second transmission lines (522, 523).
The flat antenna (51) is mounted in the casing (50) and has a feed point (A) and a ground point (B). The feed point (A) and ground point (B) are respectively connected to a core conductor (61) and a braided layer (62) of the coaxial cable (60) respectively through the first and second transmission lines (522, 523). Since impedance of the flat antenna (51) and the impedance of an antenna signal processing circuit (not shown) must be matched, the antenna signal can be completely transmitted to the antenna signal processing circuit. Therefor, in addition to the impedance of the flat antenna (51) and the antenna signal processing circuit, impedance of the first and second transmission line (522, 523) has to been considered.
Digital televisions have different sizes, and different sizes of digital receiving antenna devices are required. With further reference to FIG. 6, a conventional large passive digital antenna device has a large casing (50a), a large flat antenna (51a) and a long first transmission line (522a) and a second transmission line (523). The large flat antenna (51a) has the same impedance as the smaller flat antenna (51a) as shown in FIG. 5.
The flat antenna (51a) is a distance from the coaxial cable (60), which is longer than that of the smaller digital receiving antenna device. Consequently, a longer first transmission line (522a) is required to connect between the large flat antenna (51a) and the external coaxial cable (60). Therefore, a new impedance of the longer first transmission line (522a) is generated and the antenna signal processing circuit is not adapted to use the large antenna device since the impedance no longer matches. In brief, the large rectangular antenna device needs to use a tailored antenna signal processing circuit.
Since different rectangular passive antenna devices do not use the same antenna signal processing circuit, fabricating cost of the passive digital antenna device will be increased.
The present invention provides a digital receiving antenna device that has a fixed impedance to overcome the problem with mismatched impedance with the antenna signal processing circuit in different size digital receiving antenna devices.